In prior art methods, the feed of chips to the rotating thermomechanical pulp refiner was under manual control of set values for chip entry and water feed. In the manual control method, control delay of some process set values is naturally extremely long, typically in the order of several hours. The selection of set values is approximate and inconsistent. Furthermore, since the density and size variation of entering chips is random, this method is incapable of producing a consistent quality of thermomechanical pulp.
Efforts have been made to estimate the water quantity of the chips entering the refiner, but approaches to a reliable moisture content sensor for chips have been unsuccessful.
Also, adjustable systems are known in the art, in which an attempt has been made to maintain power consumed by the refiner constant by regulating the quantity of fed water. Yet, although the power input from the mains to the thermomechanical pulp refiner is maintained constant, thermomechanical pulp presents deviations due to variations in density of chips.
Water feed is also controlled by first measuring the freeness value of thermomechanical pulp which gives a standardized measure for pulp drainage and is characteristic of the fines of thermomechanical pulp, and then, on the basis of determined freeness, adjusting water quantity, production capacity, and disc clearance to obtain desired freeness value. This method, however, is insensitive to changes in consistency and density. In addition, freeness measurement is time consuming, and consequently, does not lend to real-time control, but rather presents an appreciable delay between the time of measurement to that of control.